Metals from Groups I and II of the periodic table are commonly used to initiate the polymerization of monomers into polymers. For example, lithium, barium, magnesium, sodium, and potassium are metals that are frequently utilized in such polymerizations. Initiator systems of this type are of commercial importance because they can be used to produce stereo regulated polymers. For instance, lithium initiators can be utilized to initiate the anionic polymerization of isoprene into synthetic polyisoprene rubber or to initiate the polymerization of 1,3-butadiene into polybutadiene rubber having the desired microstructure.
The polymers formed in such polymerizations have the metal used to initiate the polymerization at the growing end of their polymer chains and are sometimes referred to as living polymers. They are referred to as living polymers because their polymer chains which contain the terminal metal initiator continue to grow or live until all of the available monomer is exhausted. Polymers that are prepared by utilizing such metal initiators normally have structures which are essentially linear and normally do not contain appreciable amounts of branching.
Rubbery polymers made by living polymerization techniques are typically compounded with sulfur, accelerators, antidegradants, a filler, such as carbon black, silica or starch, and other desired rubber chemicals and are then subsequently vulcanized or cured into the form of a useful article, such as a tire or a power transmission belt. It has been established that the physical properties of such cured rubbers depend upon the degree to which the filler is homogeneously dispersed throughout the rubber. This is in turn related to the level of affinity that filler has for the particular rubbery polymer. This can be of practical importance in improving the physical characteristics of rubber articles which are made utilizing such rubber compositions. For example, the rolling resistance and traction characteristics of tires can be improved by improving the affinity of carbon black and/or silica to the rubbery polymer utilized therein. Therefore, it would be highly desirable to improve the affinity of a given rubbery polymer for fillers, such as carbon black and silica.
In tire tread formulations better interaction between the filler and the rubbery polymer results in lower hysteresis and consequently tires made with such rubber formulations have lower rolling resistance. Low tan delta values at 60° C. are indicative of low hysteresis and consequently tires made utilizing such rubber formulations with low tan delta values at 60° C. normally exhibit lower rolling resistance. Better interaction between the filler and the rubbery polymer in tire tread formulations also typically results higher tan delta values at 0° C. which is indicative of better traction characteristics.
The interaction between rubber and carbon black has been attributed to a combination of physical absorption (van der Waals force) and chemisorption between the oxygen containing functional groups on the carbon black surface and the rubber (see D. Rivin, J. Aron, and A. Medalia, Rubber Chem. & Technol. 41, 330 (1968) and A. Gessler, W. Hess, and A Medalia, Plast. Rubber Process, 3, 141 (1968)). Various other chemical modification techniques, especially for styrene-butadiene rubber made by solution polymerization (S—SBR), have also been described for reducing hysteresis loss by improving polymer-filler interactions. In one of these techniques, the solution rubber chain end is modified with aminobenzophenone. This greatly improves the interaction between the polymer and the oxygen-containing groups on the carbon black surface (see N. Nagata, Nippon Gomu Kyokaishi, 62, 630 (1989)). Tin coupling of anionic solution polymers is another commonly used chain end modification method that aids polymer-filler interaction supposedly through increased reaction with the quinone groups on the carbon black surface. The effect of this interaction is to reduce the aggregation between carbon black particles which in turn, improves dispersion and ultimately reduces hysteresis.
U.S. Pat. No. 4,935,471 (to Adel F. Halasa et al.) discloses a means for capping living polydiene rubbers in order to improve their affinity for carbon black. The capped rubbery polymers made by this technique are reported to be useful in manufacturing tire treads which have lower level of rolling resistance and better traction characteristics. This patent more specifically discloses polydiene rubber having a high level of affinity for carbon black which is comprised of polymer chains having repeat units which are derived from at least one conjugated diolefin monomer wherein said polymer chains are terminated with a member selected from the group consisting of cyanide groups and heterocyclic aromatic nitrogen containing groups. U.S. Pat. No. 4,935,471 also reveals a process for preparing a polydiene rubber having a high level of affinity for carbon black which comprises reacting a metal terminated polydiene with a capping agent selected from the group consisting of (a) halogenated nitrites having the structural formula X-A-C≡N wherein X represents a halogen atom and wherein A represents an alkylene group containing from 1 to 20 carbon atoms, (b) heterocyclic aromatic nitrogen containing compounds, and (c) alkyl benzoates.
Perhaps more interesting from a chemistry perspective are the persistent literature references to rubber “additives” that improve vulcanizate properties such as fatigue, dynamic modulus and hysteresis loss by modifying the polymer-filler interaction (see A Zyusin et al., Intenat. Poly. Sci. & Tech., 11 (2), T/56 (1984); A Payne et al., J. Rubber Res. Inst. Malaya, 22, 275 (1969); H Leeper et.al., Rubber World, 135, 413 (1956); A Lykin et.al., Rubber Chem. & Technol., 46, 575 (1973); A Klasek et al., J. Applied Poly. Sci., 61, 1137 (1996); V. Strygin et al., Internat. Poly. Sci. & Tech., 24 (3), T/14 (1997); K Tada et.al., J. Applied Poly. Sci., 15, 117 (1971); D Graves, Rubber Chem. & Technol., 66, 62 (1993); and L Gonzalez et al., Rubber Chem. & Technol., 69, 266 (1996)).
One of the first additives seen to have such an effect was p-nitrosodiphenylamine (PNDPA). This material was originally developed by scientists from the Natural Rubber Producers Research Association, (NRPRA), for its antioxidant activity and its ability to chemically bind to the polyisoprene structure through the nitroso group (see A Payne et al., J. Rubber Res. Inst. Malaya, 22, 275 (1969). Subsequent extensive work by Russian scientists, however, discovered that polyisoprene modified with PNDPA during mixing ultimately decreases hysteresis loss in vulcanizates and improves the green strength of the mixes. While it was clear from bound rubber measurements and other techniques that there was a preferential adsorption of the modified macromolecules on the surface of the carbon black, the nature of the bonding could not be clearly determined with the analytical tools of the time.
Nitrones are useful intermediates in a wide variety of applications. For example, nitrones are important as intermediates in organic synthesis, particularly in [3+2] cyclo addition reactions. Nitrones are excellent 1,3-dipoles and capable of reacting with double and triple bonds to form 5-membered heterocyclic ring structures. For example, isoxazolines and isoxazoles are formed by reacting nitrones with carbon-carbon double and triple bonds respectively. Accordingly, nitrones have been utilized for synthesizing various nitrogen containing biologically active compounds, for example, antibiotics, alkyloids, amino sugars, and beta-lactams.
In addition, nitrones are also known for their ability to act as efficient free radical “spin traps”. Nitrones behave as spin trapping agents when a diamagnetic nitrone (the spin trap) reacts with a transient free radical (having a spin) to provide a more stable free radical (referred to as the spin adduct). More specifically, a very reactive oxygen-centered or carbon-centered free radical reacts with the nitrone to generate a new and very stable nitroxide radical adduct. The radical adduct generated may be detectable by electron para-magnetic resonance (EPR) spectroscopy if the stabilized free radical has a reasonable lifetime. Further, information about a spin of a radical can be gleaned from a study of the structure and spectroscopic characteristics of the new radical adduct due to the increased radical stability and lifetime. Thus, techniques utilizing nitrone spin trapping agents are an important method for garnering information on free radicals otherwise difficult or impossible to detect by direct spectroscopic observation due to their exceedingly short lifetimes and low concentrations.
Techniques utilizing nitrone spin trapping agents are also useful for studying free radical responses in biological systems. For example, the toxicity of a synthetic beta amyloid peptide preparation towards glutamine synthesis could be correlated with the characteristics of an EPR signal generated by the spin adduct formed from each batch of synthetic beta amyloid peptide and spin trap. U.S. Pat. No. 6,107,315 discloses the use of a spin trapping reagent, such as α-phenyl-N-tert-butyl nitrone (PBN), in a suitable pharmaceutical carrier for administration to a patient for the treatment of symptoms associated with aging or other conditions associated with oxidative tissue damage. U.S. Pat. No. 5,723,502 discloses a method for ameliorating a cellular dysfunction of a tissue, such as the cosmetic treatment of hair loss and stimulation of hair growth, by administering a nitrone spin trap, such as PBN, to the affected tissue.
Nitrones have also been found to be useful as agents for controlled free radical polymerization. More specifically, the presence of a stable nitrone free radical during the polymerization or copolymerization of monomers provides for control of polymerization and results in polymers having a relatively narrow polydispersity, relative to polymers formed in the absence of a stable nitrone free radical. For example, U.S. Pat. No. 6,333,381 discloses the use of PBN to control the polymerization used in the synthesis of various types of rubbers.